Manufacturing semiconductor devices involves depositing and patterning several overlaying layers. A typical semiconductor wafer might include, for example, a series of gates formed on a first layer and a series of interconnects formed on a second layer. The two layers (and their structures) are formed at different lithography steps in the manufacturing process. Alignment between the two layers is critical to ensure proper connection between the gates and their interconnects. Typically, this means that the tolerance of alignment must be less than the width of a single gate.
Overlay is defined as the displacement of a patterned layer from its ideal position aligned to a layer patterned earlier on the same wafer. Overlay is a two dimensional vector (Δx, Δy) in the plane of the wafer. Overlay is a vector field, i.e., the value of the vector depends on the position on the wafer. Perfect overlay and zero-overlay are used synonymously. Overlay and overlay error are used synonymously. Depending on the context, overlay may signify a vector or one of the components of the vector.
Overlay metrology provides the information that is necessary to correct the alignment of the stepper-scanner and thereby minimize overlay error with respect to previously patterned layers. Overlay errors, detected on a wafer after exposing and developing the photoresist, can be corrected by removing the photoresist, repeating exposure on a corrected stepper-scanner, and repeating the development of the photoresist. If the measured error is acceptable but measurable, parameters of the lithography process could be adjusted based on the overlay metrology to avoid excursions for subsequent wafers.
Most prior overlay metrology methods use built-in test patterns etched or otherwise formed into or on the various layers during the same plurality of lithography steps that form the patterns for circuit elements on the wafer. One typical pattern, called “box-in-box” consists of two concentric squares, formed on a lower and an upper layer, respectively. “Bar-in-bar” is a similar pattern with just the edges of the “boxes” demarcated, and broken into disjoint line segments. The outer bars are associated with one layer and the inner bars with another. Typically one is the upper pattern and the other is the lower pattern, e.g., outer bars on a lower layer, and inner bars on the top. However, with advanced processes the topographies are complex and not truly planar so the designations “upper” and “lower” are ambiguous. Typically they correspond to earlier and later in the process. The squares or bars are formed by lithographic and other processes used to make planar structures, e.g., chemical-mechanical planarization (CMP).
In one form of the prior art, a high performance microscope imaging system combined with image processing software estimates overlay error for the two layers. The image processing software uses the intensity of light at a multitude of pixels. Obtaining the overlay error accurately requires a high quality imaging system and means of focusing the system. One requirement for the optical system is very stable positioning of the optical system with respect to the sample. Relative vibrations blur the image and degrade the performance. Reducing vibration is a difficult requirement to meet for overlay metrology systems that are integrated into a process tool, like a lithography track.
As disclosed in U.S. Patent Application Serial No. 2002/0158193; U.S. Patent Application No. 2003/0190793 A1; and as described in Proc. of SPIE Vol. 5038 February 2003 “Scatterometry-Based Overlay Metrology” by Huang et al., p. 126–137 and “A novel diffraction based spectroscopic method for overlay metrology” by Yang et al. p. 200–207 (all four incorporated in this document by reference) one approach to overcome these difficulties is to use overlay metrology targets that are made of a stack of two diffraction gratings as shown in FIG. 1. The grating stack 10 has one grating 20 in the lower layer and another grating 30 in the upper layer as shown in FIG. 1. The layers of 20 and 30 are to be aligned. There are two instances of the grating stack 10, one for the x-component of overlay and one for the y-component. The measurement instrument is such that it does not resolve individual grating lines. It measures overall optical properties of the entire grating. Optical properties are measured as a function of wavelength, polar or azimuthal angle of incidence, polarization states of the illumination and the detected light, or any combination of these independent variables. An alternative embodiment uses two stacks of line gratings to measure x-overlay and two stacks of line gratings to measure y-overlay (four grating stacks total). Still another embodiment uses three line grating stacks in combination to simultaneously measure both x and y alignment. (See also PCT publication WO 02/25723A2, incorporated herein by reference). Scatterometry (diffraction) is proving to be an effective tool for measuring overlay.
A shortcoming of the prior scatterometry-based art is that, diffraction gratings cannot distinguish overlay values that differ by an integer number of periods. Let R(λ,θ,ξ) denote the specular (0-th order) reflection of the grating at wavelength λ, angle of incidence θ, and offset ξ. The offset ξ is the distance between centerlines of lower and upper grating lines as shown in FIG. 1. The function R(λ,θ,ξ) is periodic with respect to the offset ξ:R(λ,θ,ξ)=R(λ,θ,ξ+P)  (1)
where P is the period of the grating. The scatterometry-based overlay measurements are even more ambiguous when the profiles of the grating lines are symmetric. Then, a consequence of reciprocity is:R(λ,θ,ξ)=R(λ,θ,−ξ)  (2)R(λ,θ,(P/2)+ξ)=R(λ,θ,(P/2)−ξ)  (3)
There are two values of offset, separated by P/2, where the optical properties become ambiguous. Optical properties are insensitive to overlay at these points. Therefore, the largest measurement range is P/2 when the grating lines have symmetric cross-sections. If the unit cell of the grating is substantially asymmetric, the measurement range becomes P, a whole period. Huang et al., cited above, describe means of manufacturing a grating with an asymmetric unit cell. The measurement range also becomes a whole period if two grating stacks are used, the offset of each stack is biased, and the two offset biases differ by P/4 (as described in U.S. application Ser. No. 10/613,378, filed Jul. 3, 2003, incorporated in this document by reference).
FIG. 2 shows the calculated near-normal incidence, unpolarized reflectance of a grating stack as a function of offset ξ. Each curve corresponds to a different wavelength. The ambiguity described by Equations (2) and (3) is clearly visible in FIG. 2. The ordinate of the graph in FIG. 3 shows overlay measured using diffraction gratings with a 1 μm period and 1:1 line to space ratio. Overlay was obtained by fitting the reflection spectra of a pair of gratings with rigorous coupled wave analysis. The abscissa shows the best estimate of the actual value of overlay, which is the sum of the overlay that was intentionally written to the reticle that produced the gratings and the overlay that was imparted by the lithography stepper-scanner. Overlay measured using gratings is not valid when it approaches or exceeds ±P/4 (250 nm in this example). FIG. 4 is an abstraction that shows the periodic nature of the overlay measured using diffraction gratings. The overlay measured by the grating, ΔxGRATING, is related to the actual overlay as follows:
                                                        N              =                              int                ⁡                                  [                                                            Δ                      ⁢                                                                                          ⁢                                              x                        ACTUAL                                                                                    P                      /                      2                                                        ]                                                                                                                        Δ                ⁢                                                                  ⁢                                  x                  GRATING                                            =                                                                    (                                          -                      1                                        )                                    N                                ⁡                                  [                                                            Δ                      ⁢                                                                                          ⁢                                              x                        ACTUAL                                                              -                                          N                      ⁢                                                                                          ⁢                                              P                        /                        2                                                                              ]                                                                                        (        4        )            
In Equation (4), int[x] denotes the integer nearest to x. In prior art, the integer N is unknown. Therefore, ΔxGRATING represents ΔxACTUAL only when N is zero, that is, when |ΔxACTUAL|<P/4. ΔxGRATING, which is also called fine-overlay measurement, has high precision but it is only accurate when N=0. Gross overlay is defined as the condition |ΔxACTUAL|≧P/4 or |ΔyACTUAL|≧P/4, that is, any one component of overlay exceeding a quarter of the period. Gratings cannot detect gross overlay until overlay gets so large that the upper and lower gratings do not overlap in part of the measurement spot. Although gross overlay is rare in well-tuned lithography processes, alignment errors larger than 100 nm, even as large as several microns occur when a new process, a new reticle, or a new projector is introduced. In these instances, there is a need to not only detect but also to measure gross overlay.
Although increasing the period increases the measurement range, P/2, this approach is not preferred because it reduces the sensitivity of the optical response of grating stacks to overlay. As the period is increased, if the period becomes a significant fraction of the diameter of the measurement spot, the placement of the spot on the grating affects the overlay measurement and reduces its precision. For these two reasons, increasing the period to increase the measurement range is counter-productive. Using more than one grating stack, each with a different period, reduces but does not eliminate ambiguity.